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Vocabulary Word
Definition
CHAPTER 12
the branch of science that involves observations and explanations of
Astronomy events and objects that occur beyond Earth and its atmosphere
a scientist who studies astronomy; different astronomers study
Astronomer
different aspects of astronomy
Rotation the spinning of an object around an imaginary line called an axis
the motion of an object in a circular or elliptical path (usually around
Revolution another object)
Photoperiod the number of hours of daylight between sunrise and sunset
Planet a large object that orbits the Sun
the apparent slowing, reversal, and then
Retrograde motion looping of a planet in its path across the sky
Precession the changing direction of Earth’s axis
the point at which Earth is closest to the Sun; occurs around
Perihelion January 3
the point at which Earth is farthest from the Sun;
Aphelion
occurs around July 4
Constellation a group of stars that form a pattern
an imaginary rotating sphere upon which lie the all the objects
Celestial sphere in the universe
a circle on the celestial sphere; the extension of Earth’s
Celestial equator
equator projected into the universe
the apparent path the Sun takes through the
Plane of the ecliptic sky, as marked by the 12 constellations of the zodiac
the two dates when the Sun, on the ecliptic, crosses the celestial
equator; occurring around March 21 and September 22, and known as the
Equinoxes
spring and fall equinoxes, respectively. On these days, the number of hours
the Sun is above the horizon and below the horizon is equal.
occur when the Sun reaches its highest and lowest positions in the
sky, when Earth is tilted closer to or farther away from the Sun (due to the
Solstices 23.5° axis tilt). The summer solstice usually occurs on June 21, marking the
first day of summer and the longest day of the year. The winter solstice
usually
occurstoonconstellations
December 21around
and marks
the beginning
of winter and that
the shortest
refers
the pole;
refers to constellations
Circumpolar never
day of disappear
the year. below the horizon as Earth rotates
refers to the Moon’s phases, which can be divided into eight distinct
stages
results during a new moon, when the Moon, at one of its nodes, is
Solar eclipse
directly between Earth and the Sun
Lunar cycle
Lunar eclipse
Nodes
Tides
Nebula
results when Earth is positioned between the Sun and the Moon
such that Earth casts its shadow on the surface of the Moon; the Moon must
be at a node
the points at which the Moon’s orbital plane intersects Earth’s orbital plane;
eclipses occur only at lunar nodes
the alternate rising and falling of the surface of large bodies of water; caused
by the interaction between Earth, the Moon, and the Sun
refers to a collection of dust and gases, consisting mainly of hydrogen
and helium gases and, to a lesser extent, grains of solid matter such as iron,
rock, and ice
small clumps of matter, forced together by gravity, that condense to
form a planet
the name given to the four planets beyond Mars: Jupiter, Saturn,
Outer planets Uranus, and Neptune; also known as the outer planets; their atmospheres
consist mainly of hydrogen and helium gases
Protoplanet
the name given to the four planets beyond Mars: Jupiter, Saturn,
Gas giants Uranus, and Neptune; also known as the outer planets; their atmospheres
consist mainly of hydrogen and helium gases
the name given to the four planets closest to the Sun:
Mercury, Venus, Earth, and Mars; also known as the terrestrial planets. They
Inner planets resemble Earth in that they are small and have densities similar to most
rocks, about 5 g/cm3
the name given to the four planets closest to the Sun:
Mercury, Venus, Earth, and Mars; also known as the inner planets. They
Terrestrial planets resemble Earth in that they are small and have densities similar to most
rocks, about 5 g/cm3
refers to the non-planetary matter in the solar system; includes
asteroids, meteoroids (meteors and meteorites), and comets
rocky and metallic object that revolves around the Sun but is too small
Asteroid to be called a planet; sometimes called minor planets, most are settled in the
asteroid belt
Meteoroid lump of rock or metal that encounters Earth as it travels through space
smaller chunk of solid matter that burns up in Earth’s atmosphere due to
Meteor friction, creating a bright streak of light; often called a shooting star
Minor bodies
Meteorite larger meteor that actually reaches Earth’s surface
chunk of frozen matter that orbits the Sun in very long elliptical paths;
Comet some visit the Sun once; come from the Oort Cloud and the Kuiper Belt
the current theory that explains the formation of the Moon; it
states that, approximately 4.5 billion years ago, the Moon was formed as a
Big Splash Theory
result of an object approximately the size of Mars colliding with Earth,
causing less dense portions to be blasted into space
Astronomical Unit defined as the distance from Earth to the Sun, 150 million kilometres
Astrolabe a historical instrument used by astronomers to determine the distance
to stars; also used as a navigation tool
Earth-centred; refers to the Earth-centred model of the universe,
Geocentric which places a motionless Earth at the centre with all the planets and stars at
fixed positions within eight concentric spheres that spin in circles called orbits
Sun-centred; refers to the Sun-centred model of our solar system,
Heliocentric in which all planets revolve around the Sun
three laws that state:
(1) the planets travel in elliptical orbits around an off-centre Sun, with the Sun
Kepler’s Laws of Planetary at one focal point;
Motion (2) the speed of a planet’s orbit depends on its distance from the Sun;
(3) the farther a planet (or dwarf planet) is from the Sun, the longer its orbit.